PHASE-TRANSITIONS IN MOLECULAR-CRYSTALS FROM A CHEMICAL VIEWPOINT

Why do molecules pack in crystals as they do? The trivial answer: because they thereby achieve periodic arrangements corresponding to minimum potential energy. In general, for any given molecule, there must be many such arrangements of closely similar energy; experience accumulates to show that polymorphism is of widespread occurrence in the world of molecular crystals. It is often accompanied by radical changes in conformation and hydrogen bonding patterns. Indeed, as far as molecular crystals are concerned, there seems to be no clear cut boundary between polymorphic transitions and solid state chemical reactions. Of particular interest are those cases where crystal orientation is preserved during the structural change, for there, in principle, the atomic positions can be mapped quite precisely on both sides of the transition and mechanisms can be proposed for the molecular motions that are involved. Not all such processes occur under topochemical control, i.e., with a minimum amount of atomic or molecular movement from the known starting structure. Many seem to occur, or at least to be initiated, at crystal defects where the regular arrangement of molecules is interrupted. Examples of both types will be discussed, and the attempt will be made to describe these and related phenomena from a chemical point of view.